1. Field of the Invention
The invention relates to an abnormality determination apparatus and abnormality determination method for a multi-cylinder internal combustion engine equipped with a dual-injection system having two fuel systems for injecting and supplying fuel to each of cylinders and which determine an abnormal variation in air-fuel ratio among the cylinders.
2. Description of Related Art
In a multi-cylinder internal combustion engine, a variation in air-fuel ratio among cylinders may occur because of a variation in fuel injection amount among the injectors of the cylinders. When such a variation in air-fuel ratio among the cylinders occurs, there are the cylinders of which the air-fuel ratio is richer than the stoichiometric air-fuel ratio and the cylinders of which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio even when the total air-fuel ratio of the internal combustion engine coincides with the stoichiometric air-fuel ratio.
When such a variation in air-fuel ratio among the cylinders occurs, the air-fuel ratio detected from the oxygen concentration of exhaust gas fluctuates, and the rate of change in air-fuel ratio increases. In addition, when a variation in air-fuel ratio occurs, a variation in generated torque among the cylinders occurs, and the amount of fluctuation in engine rotation speed increases. Therefore, it is possible to determine whether there is an abnormal variation on the basis of an accumulated value of the absolute value of the rate of change in air-fuel ratio or an accumulated value of the amount of fluctuation in engine rotation speed during a predetermined determination period.
Incidentally, in recent years, a multi-cylinder internal combustion engine equipped with a dual-injection system that includes two fuel systems for injecting and supplying fuel to each cylinder is practically used. A dual-injection system of this type includes a port injection fuel system that injects fuel into intake ports and a direct injection fuel system that directly injects fuel into cylinders. In the multi-cylinder internal combustion engine equipped with such a dual-injection system, a P injection mode in which only port injection is carried out, a D injection mode in which only direct injection is carried out and a PD injection mode in which both port injection and direct injection are carried out are used on the basis of an engine operating condition.
Then, the apparatus described in Japanese Patent Application Publication No. 2010-169038 (JP-A-2010-169038) has been suggested as an abnormality determination apparatus that determines whether there is an abnormal variation in air-fuel ratio among the cylinders of a multi-cylinder internal combustion engine equipped with a dual-injection system. The abnormality determination apparatus described in JP-A-2010-169038 separately accumulates a determination parameter for each injection mode and also makes an abnormality determination on the basis of the accumulated value for each injection mode.
The above abnormality determination apparatus estimates that there is an abnormality in the port injection fuel system when the accumulated value of the determination parameter for the P injection mode indicates an abnormal value, and estimates that there is an abnormality in the direct injection fuel system when the accumulated value of the determination parameter for the D injection mode indicates an abnormal value. Then, when any one of the fuel systems is estimated to have an abnormality, the injection mode is fixed in the mode in which only the fuel system estimated to have an abnormality is used and then an abnormality determination is made to thereby finally determine the fuel system that has an abnormality. Note that in such an existing abnormality determination apparatus for a multi-cylinder internal combustion engine, the accumulated value of the determination parameter for the PD injection mode is used only in order to insure the accuracy of the above estimation.
In the above technique, the fuel system that may have an abnormality cannot be identified unless the P injection mode and the D injection mode each are carried out for a set period of time or longer irrespective of whether they are intermittently carried out. However, depending on the operation of the multi-cylinder internal combustion engine, there may be a long time and many opportunities that the PD injection mode is carried out, and there may be limited time and opportunities that the P injection mode or the D injection mode is carried out. In such a case, with the above technique, it may take a great deal of time to make an abnormality determination.
Note that such a problem can also similarly occur in, for example, a system that includes a duel-injection system, other than a system that includes one of each of a port injection fuel system and a direct injection fuel system, such as a system that includes two direct injection fuel systems for each cylinder.